Cationic ligands

For both catalytic and stoichiometric reactions, each step of the process taking place on the metal can be influenced by the nature of ligands, cations, anions, or solvent. The effects of these factors on reaction rate, selectivity, stereoselectivity, etc. cannot be easily predicted, because each step can be influenced in different ways. The reader is referred to the literature cited below. 

The stability of crown-ether complexes depends on several factors these include cavity size of the ligand, cation diameter, spatial distribution of ring binding sites, the character of the hetero-atoms, the presence of additional binding sites and the type of solvent used. In apolar solutions it also depends on the nature of the anion. The effects of these parameters will be illustrated in the next sections. 

Rates of decomplexation (kJ2) of cation complexes can also be determined by nmr spectroscopy on the cation. Rates of complex formation are then calculated from kn and the binding constant. The results for several ligands, cations, and solvents are given in Table 20. Despite the wide variations, the rates of complex formation are all in the range 2 x 107 to 8 x 10 M 1 s 1. In contrast, rates of decomplexation for crown-ether complexes span a much broader range 6.1 x 102 to 2 x 105 s 1. Comparison of crown-ether data with data for [2.2.2]-cryptand [37] and the linear polyether [92] also shows that the... 

Newer synthetic acyclic substances that can be used in ISEs for divalent metal ions (usually forming 2 1 ligand/cation complexes) should meet the following conditions given by Simon and coworkers [2,3] ... 

The difference between Cs+ and K+ in the two sequences may be accounted for by the formation of 2/1 ligand/cation complexes (136). 14 is not expected to undergo important conformational changes on complexation. 

That there is a relationship between the coordination number of the metal in crystalline halogenocuprates(I) and halogenoargentates(I) and the properties of the cation with which it is coprecipitated would now seem to be well established. The tuning of anionic configurations to cation properties reflects the versatility in coordination requirements not only of the metal but also of the ligands. Cation size would appear to be of prime importance for the determination of a particular metal(I) coordination number, the tendency to attainment of a higher coordina-... 

Alcoholate Maximum combining ratio, ligand/cation ... 

The mixed-ligand cations CBr (OTeI 5)3 1 (11 1,2), which contain the highly... 

The high electrophilicity of the silicenium ion can be modified by intramolecular electron donation from remote substituents. Corriu and co-workers668 were the first to synthesize species 269 with the bidentate nitrogen ligand. Cations with other structural units and donor atoms (O, S, P) were subsequently generated.669-674 Due to the interaction called internal solvation,675 the silicon atom becomes pentacoordinated and largely loses its ionic character, that is, such ions differ in principle from the trivalent silicenium ions. 

Baaden, M., Berny, F., Madic, C., Schurhammer, R., Wipff, G. 2003. Theoretical studies on lanthanide cation extraction by picolinamides Ligand-cation interactions and interfacial behavior. Solvent Extr. IonExch. 21 (2) 199-220. 

The tetrachloronickel(II) salt of the ligand cation is readily converted to the hexafluorophosphate salt. This procedure is necessary to avoid the reinsertion of any nickel(II) into the macrocyclic ligand during the neutralization process. Ten grams of [C22H26N4] [NiCl4] is dissolved in a minimum amount (about 100 mL) of water, and 9.0 g (1.5 excess) of ammonium hexafluorophosphate, also dissolved in a minimum amount (about 8 mL) of water, is added slowly. A colorless precipitate of the hexafluorophosphate salt [C22H26N4] [PF6]2 forms immediately. The product is filtered, washed with water, and air dried. The yield is quantitative. 

The tris-ligand cation of bipy-A-oxide (120),613 and the bis-ligand cation of terpy-A-oxide314 can be prepared by electrochemical oxidation of the Mnu species in CH3CN, and the robust dark-red crystalline perchlorates have been isolated. 

With neutral ligands, cationic species such as [Ag(py)4]2+, [Ag(bipy)2]2+, and [Ag(phen)2]2+ form crystalline salts, whereas with uninegative chelating ligands such as 2-pyridinecarboxylate, neutral species such as (18-I-IX) are obtained. In acid solution the [Ag(bipy)2]2+ ion is in equilibrium with the [Ag(bipy)]2+ ion, and it is the latter that oxidizes H202 in acid solution. 

FIGURE 3. Stereo model of the pAPX cation site. Oxygen atoms and the cation are gray, and nitrogen atoms are black. The thin lines highlight ligand-cation interactions. Reprinted with permission from Bonagura et al., 1996. 

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